Polymers are often used in medical device applications due to the fact that they are easy to process, they have good mechanical properties, and they have an acceptable level of biocompatability. In certain medical device applications, the surface of the polymer will be in contact with the cells and fluids of the body. In these applications, it is necessary that the surface of the polymer have certain beneficial properties in order to be in contact with the cells and fluids of the body. In certain applications and with certain polymers, the surface of the polymer will not embody these certain beneficial characteristics. In such cases, it may be necessary to modify a thin surface layer of the polymer so that it may embody these beneficial characteristics, while at the same time maintaining the beneficial characteristics of the bulk properties of the polymer.
Numerous types of polymer surface modifications and methods for preparing such modifications are known. Such existing surface modification methods on polymers include gas plasma, radiation grafting, photoinduced grafting, sol-gel process, surface etching, and polyamine adsorption. Although these existing surface modification techniques are adequate for their purposes, they each have their drawbacks. For instance, the gas plasma technique tends to yield non-uniform surfaces on polymers such as silicone, which would increase, rather than reduce, cell adhesion. Ionizing radiation may weaken and discolor the polymer material, which is a significant drawback with silicone intraocular lenses. In photoinduced grafting, a coupling agent reacts directly with the polymer substrate surface, and thus the process is sensitive to the particular reactive groups located on the surface of the polymer substrate. The sol-gel process creates a modified surface that lacks long-term stability. Surface etching cleaves the polymer backbone, which may weaken the surface structure of the polymer.
Another method of making a surface modification to a polymer is to chemically bond a coupling agent directly to the surface of the polymer. This method requires that the coupling agent bond directly to a reactive group on the surface of the polymer substrate. Thus, because the coupling reaction is dependent on the reactive groups located on the surface of the polymer substrate, the particular reaction conditions and coupling agents will be dependent on the particular reactive group.
Another known surface modification technique is the use of bulk interpenetrating polymer networks. An interpenetrating polymer network is a combination of two polymers in the network form, at least one of which is polymerized in the presence of the other. Bulk interpenetrating polymer networks are synthesized by polymerization throughout the bulk of the polymer. For instance, European Patent No. 643,083 discloses a bulk interpenetrating polymer network from polydimethylsiloxane and polyacrylic acid for fabricating soft contact lenses. Further, U.S. Pat. No. 5,426,158 discloses contact lenses made from a bulk interpenetrating polymer network.
The bulk interpenetrating polymer networks, although adequate for their intended use, have drawbacks of their own. For instance, in bulk interpenetrating polymer networks, the polymerization initiator is uniformly distributed throughout the bulk of the polymer to be modified. Polymers that have bulk interpenetrating polymer networks formed therein have different physical properties than the same polymer without the bulk interpenetrating polymer networks, because networks are created throughout the entirety of the polymer. Thus, for example, an untreated clear polymer that has a bulk interpenetrating polymer network may be cloudy throughout, and thus not suitable for an optical application. Further, in polymers having bulk interpenetrating polymer networks, the functional monomers used in the polymerization process are mixed into the entire bulk of the polymer, and therefore only part of these molecules are available for functioning on the surface of the polymer.
Accordingly, it will be appreciated from the foregoing that there is a definite need for a process whereby the surface of a polymer can be modified to have certain desired properties, while at the same time maintaining the desirable physical properties of the polymer. One of the desired properties resulting from the surface modification should be the ability of the surface of the polymer to couple with a surface modification agent, such as heparin. The process should provide for an interpenetrating polymer network which occurs only on the surface of the polymer, rather than in the bulk of the polymer. The coupling agent of the interpenetrating polymer network should not be chemically bound to any reactive groups on the polymer substrate so that the process of forming the surface modification is not sensitive to the particular reactive groups located on the polymer substrate. Further, the process should yield uniform surfaces on the polymer, should not weaken or discolor the polymer material, should be relatively simple and inexpensive, and should provide a surface modification that has long-term stability. The present invention meets these needs.